The chemical composition of the Earth’s mantle is a lot more variable and diverse than previously thought, a new study has revealed.
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| A mineral map of a cumulate mineral sample [Credit: Sarah Lambart/ University of Utah] |
According to a new analysis of cores drilled through the ocean crust, the mantle is made up of distinct sections of rock each with different chemical make-ups.
The chemical composition of the mantle has been notoriously difficult to determine with a high degree of certainty because it is largely inaccessible.
Scientists have traditionally relied on lava that erupts on the ocean floor to give them some idea of what the mantle is made up of, and so far studies have suggested that it’s chemically mostly the same everywhere on the planet.
However in their new study, the team of researchers led by scientists at Cardiff University have studied the very first minerals that begin to form when lava first makes contact with the crust at mid-ocean ridges.
In particular they looked at variations in isotopes of neodymium and strontium, which can indicate different chemistries of mantle material that come from different types of rocks.
Results showed that the amount of isotope variability in the minerals was seven times greater than that in the mid-ocean ridge lavas.
The team believe that this variability could be linked to the process whereby old ocean crust spreads away from mid-ocean ridges until it’s shoved beneath a continent and sinks back into the mantle.
«Our results show that recycled oceanic crust is much more abundant in the mantle than we thought,» said principal investigator of the study Dr Johan Lissenberg from Cardiff University’s School of Earth and Ocean Sciences.
«This recycled crust melts again, along with the surrounding mantle, but the chemically different melts remain isolated during their transport to the crust. Only after they reach magma chambers in the crust do they mix to produce the homogeneous lavas we observe on the seafloor.»
So rather than a gradient of yellow and orange that we regularly see represented as the mantle in textbooks, the true picture could be quite different.
«If you look at a painting from Jackson Pollock, you have a lot of different colours,» said lead author of the study Dr Sarah Lambart, who undertook the work at Cardiff University and is now based at the University of Utah.
«Those colours represent different mantle components and the lines are lava produced by these components and transported to the surface.»
The study has gone some way to helping the scientists understand the processes that are taking place below our feet, and trying to explain the chemical composition of the rocks that we see today.
The study has been published in the journal Nature Geoscience.
Source: Cardiff University [May 20, 2019]
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